First responders, respiratory therapists and critical care personnel perform emergency laryngoscopy and intubation under a variety of conditions and under great duress. Securing a viable and protected airway is one of the paramount steps of a successful resuscitation. Often times airway manipulation and instrumentation are performed in suboptimal conditions by inexperienced or lightly trained personnel. These procedures have the potential for disaster if they result in an esophageal intubation, causing hypoxia, anoxia, and cardiopulmonary arrest if allowed to continue unrecognized.
Capnography, the measurement of CO2 in expired or respirated gases has been commonly used in the operating room setting for several years. Capnography readily identifies situations that can lead to hypoxia if left undetected and dealt with. For example, one use of a CO2 measuring device is to confirm proper endotracheal tube placement during general anesthesia. By identifying improper placement, the provider can then rectify potential hypoxic conditions before hypoxia can actually lead to severe brain damage. Recently the use of capnography has been extended outside of the operating room arena to include emergency rooms, intensive care units, endoscopic suites, radiographic suites and first responders at catastrophic events (e.g. motor vehicle or industrial accidents).
The current standard of care for collect endotracheal tube placement calls for multiple methods of confirmation, one of which could be a carbon dioxide detector. Typically, however, the method used to confirm proper placement is a capnographic waveform monitor. Unfortunately, this monitor may be a complex electronic device only capable of functioning in highly controlled environments, such as an operating room. In many cases, these devices are not available, suited, or adapted for the location in which these procedures may be necessary.
Other types of endotrachael tube placement confirmation may be a disposable colorimetric detector. This type of detector confirms the presence of CO2 via a visible color change in equipment or test strip when exposed to exhaled gases containing CO2. This device detects CO2 via a chemical reaction which causes a color shift in a reagent containing substrate contained within the device.
Colorimetric detectors are generally useful as qualitative indicators of the presence or absence of CO2. Various methods have been disclosed for quantitative detection of CO2 in respired gas samples. However limitations of these devices may be that they may not provide useful feedback during various patient procedures such as cardiopulmonary resuscitation and/or ventilation. These simple detectors may not add value to patient outcomes beyond informing a simple gate decision of whether CO2 is present or absent in respiratory gases.
CO2 concentration at the end of a breath can represent the end tidal carbon dioxide concentration (PETCO2). Decreases in cardiac output and pulmonary blood flow can result in decreases in PETCO2. Correspondingly, increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO2. The relationship between cardiac output and PETCO2 has been determined to be logarithmic. Therefore capnography can detect the presence of pulmonary blood flow even in the absence of major pulses, and it can indicate changes in pulmonary blood flow caused by alterations in cardiac rhythm. Initial data samples reveal that the PETCO2 may correlate with coronary perfusion pressure. This correlation between perfusion pressure and PETCO2 is likely to be secondary to the relationship between PETCO2 and cardiac output.
Capnographic measurements have been evaluated to predict outcomes in cardiac arrest. A study involving 127 patients revealed that only one patient with a PETCO2 less than 10 mm Hg during resuscitation survived to hospital discharge. In another prospective investigation involving 139 adult victims of out-of-hospital, non-traumatic cardiac arrest, no patient with an average PETCO2 less than 10 mm Hg upon initial resuscitation survived. The analysis of these studies concluded that PETCO2 can be correlated with resuscitation and outcome in cardiopulmonary resuscitation (CPR). Moreover, another application of capnography in this setting is to provide feedback to optimize chest compressions during CPR. Monitoring PETCO2 may detect inadequate chest compressions secondary to fatigue that could result in a sub-optimal cardiac output.
Capnography is gaining increasing acceptance during the resuscitation of trauma victims. PETCO2 is a marker of traumatic physiology, as it reflects changes in cardiac output. Recently a study involving 191 blunt trauma patients revealed that PETCO2 may be of value in predicting outcome from major trauma. In this investigation only 5% of patients with a PETCO2 less than 10 mm Hg survived to hospital discharge. Other studies have shown capnography to be of value in providing optimum ventilation in pre-hospital major trauma victims. Patients monitored using capnography had a statistically significant higher incidence of normoventilation (normal CO2 levels in the blood) compared to those who were not managed with capnography (63.2% vs. 20% p<0.0001).
Some previous CO2 detectors make use of an electrochemical detection device referred to collectively as “chemiresistors”. Such devices respond to the absorption of target chemical species by undergoing a change in ohmic resistance. In many chemiresistor designs, the change in ohmic resistance may provide a quantitative basis for measurement of the absorbed species. Chemiresistors may generally be comprised of an electrically insulating substrate, with at least one surface having two or more conductive electrode layers spaced apart thereon. These electrodes may comprise a metallic layer, and they may have an interdigitated geometric form. A chemiresistive layer or “ink” may cover two or more electrode layers, and act as the “absorber” that attracts the analyte species of interest. Voltage applied to the electrodes will induce a current flow within the chemiresistive ink layer. Measurement of this current may provide a quantitative basis for detection of absorbed analyte.
Absorption of a species by a chemiresistive layer results in changes in the layer's physical and/or chemical properties, resulting in a change in ohmic resistance. For example, a chemiresistive ink may comprise finely divided carbon particles in a polymeric binder. The proportion of binder and particles may be chosen such that the layer has a first ohmic resistance. Upon absorption of an organic compound having affinity for the polymeric binder, the layer may undergo swelling which causes the particles to generally move out of contact, resulting in high ohmic resistance. The change in ohmic resistance due to swelling may be in proportion to the organic compound. Heating of the layer may desorb the organic compound, regenerating the layer for a new cycle of measurement.